[asterisk-users] Why Nat=yes Nat=no Option?

Eric "ManxPower" Wieling eric at fnords.org
Wed Nov 12 23:09:34 CST 2008

Alex Balashov wrote:
> Steve Totaro wrote:
>> I have done some large installs where people are going to be in the 
>> office, sometimes out, work from home, it always changes sorta thing......
>> I have found that setting all device profiles to Nat=yes "Just Works" 
>> whether they are on the LAN or not and this is even on larger scale 
>> systems with hundreds of "phones".
>> Is there any reason why this would be frowned upon as a default?  Even 
>> to the point of, if nat= is not specified, it would default to yes?
>> Is there a performance hit somewhere, or some other downside?
>> If not, I suggest making it the default.
> The premise of nat=yes is that the domain portion of the Contact URI is 
> overridden with the real, received source IP of the request and that the 
> default expectation of port 5060 (if not specified in the Contact URI) 
> is dropped in favour of the actually received source UDP port. 
> Similarly for SDP (without SIP-aware ALG).
> I think the reason this would be frowned upon as a default is 
> philosophical in essence;  by default, per the RFC, a SIP UAC is 
> expected to behave such and such way, i.e. use the Contact URI that 
> arrives in a REGISTER request and/or INVITE.  Overriding that with the 
> received IP:port is a "hack" around prescribed behaviour, and enabling 
> hacks as default behaviour is generally considered a bad idea.

IIRC Uniden phones do not (or did not at one time) work with nat=yes if 
they were not NAT'd.

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